MIC9131BM Micrel Inc, MIC9131BM Datasheet - Page 15

• Manufacturer Part Number: MIC9131BM
• Description: IC CTRLR DC-DC PWM HV HS 16-SOIC
• Manufacturer: Micrel Inc

Specifications of MIC9131BM

• Applications: Controller, Telecom
• Voltage - Input: 9 ~ 18 V
• Number Of Outputs: 1
• Voltage - Output: 4.7 ~ 5 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 16-SOIC (3.9mm Width)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

series resistance is 10K, for a 500kHz switcher, the maximum
series resistance is 2K.
Sensing Current with a Resistor
The fast transition times of the current signal prohibit the use
of inductive resistors. Standard wire wound power resistors
will not work. Carbon composition or metal film resistors or
low inductance power resistors may be used. The overcurrent
range of the power supply and component tolerances must be
considered when selecting the current sense resistor value.
The power supply specification may call for an overcurrent
limit, which must be accounted for when selecting the cur-
rent sense resistor value. The relationship between the peak
primary current and the current sense resistor is:
where: Ip is the current in the sense resistor
The current sense resistor must not be too small or the cur-
rent sense signal will be susceptible to noise. If noise is a
problem, the current signal level should be increased.
An example is illustrated below.
The maximum peak current, I
and minimum input voltage
The maximum rms current, I
The desired current sense signal amplitude is 500mV at 1A
output current.
The current sense resistor value and power dissipation is:
A 0.5 ohm, non inductive resistor with at least a 1/2W rating
should be selected.
The series resistor is calculated to allow the 500mV-peak
signal to reach 0.82V.
The next lower value of 10kΩ is selected.
The bandwidth of the 10K resistor and the 25pF input capaci-
tance is calculated. The resistor value must be lowered if the
bandwidth is too low for the switching frequency.
The maximum switching frequency of this power supply
should be approximately six times less than the BW to pre-
vent current waveform distortion and excessive delays in
the current loop. This limits the switching frequency to the
range of 100kHz.
August 2006
MIC9131
R
I
(40µA)
R
the current sense resistor.
P
R
BW
V
R
ISNS
DISS
ISNS
ISENSE
f
SENSE
f
is the series resistor between the ISNS pin and
V
ISNS
=
is the current sourced from the ISNS pin
=
=
2
=
I
I
P
RMS
× ×
−
is the current sense resistance
V
I
×
SENSE
(
π π
SENSE
I
I
R
ISNS
2
P
ISENSE
×
×
10
R
R
1
SENSE
ISENSE
k
=
×
0.5
+
25
RMS
1
PMAX
I
ISNS
=
pF
)
=
=
=0.65A
0 65
0.5
.
×
= 1A at 120% overcurrent
0 82
=
.
R
Ω
630
2
f
40
×
− ×
kHz
0 5
(
µ
1 0 5
.
A
=
.
0 21
.
)
=
W
10 25
.
k
Ω
15
Sensing Current with a Current Sense Transformer
At higher power levels, the power dissipation in a current sense
resistor is excessive. A current sense transformer can be
used to sense the current while minimizing power dissipation.
See Figure 11. The schematic shows the circuitry necessary
when using a current sense transformer. The resistor, R1,
provides a path to reset the current sense transformer. The
resistor, R2, converts the scaled down current to a voltage,
which is sent to the ISNS pin.
The voltage at the ISNS pin is calculated by:
where: I
Current Transformer Example:
The maximum peak current, I
and minimum input voltage
The maximum rms current, I
The full 0.82V peak signal a the ISNS input can be used since
very little power is dissipation in the secondary side sense
resistor. The maximum peak to peak voltage at the sense
pin (pin 14) is 0.82V at the 5A maximum output current.
The current sense resistor value and power dissipation is:
A 16.2 ohm, 1%, non inductive resistor with at least a 50mW
rating should be selected. A good choice would be an 0805
size metal film or a 1/8 watt leaded metal film resistor. A
series resistor between the current sense transformer and
the Isns input is not necessary unless it is used for low pass
filtering.
R
P
V
transformer
R2 is the current sense resistance at the
secondary of the current sense transformer
N is the turns ratio of the current sense
transformer (N=Nsec/Npri)
I
(40µA)
R
the current sense resistor.
P
ISNS
DISS
ISNS
f
2
MIC9131
is the current in the primary of the current sense
is the series resistor between the ISNS pin and
=
=
V
is the current sourced from the ISNS pin
=



SENSE
(pin 14)
(pin 16)
I
I
N
P
PRMS
OUT
N
I
I
×
P
SNS
R

 ×

×
2
2
N
+
Rf
Figure 11
R
=
I
ISNS
2
0 82 100
=
RMS
.
PMAX
R2



×
3 25
100
V
R
5
×
.
IN
= 3.25A
f
= 5A at 120% overcurrent

 ×

2
R1
=
16 4 17 4
16 4
.
.
=
Current Sense
Transformer
Ω
I
PRI
M9999-080206
.
Micrel, Inc.
mW